The present invention relates to an M F S (Metal-Ferroelectric-Semiconductor) type semiconductor device, which has a structure in which the oxide of a MOS (Metal-Oxide-Semiconductor) type semiconductor device is replaced by a ferroelectric.
As a ferroelectric layer of the MFS type semiconductor device, an oxide ferroelectric has been used.
However, when an oxide ferroelectric is directly formed on a silicon substrate, the following problems are generated.
(1) Unnecessary film of such as SiO.sub.2 is formed on an interface between a ferroelectric and a semiconductor substrate.
Nevertheless, in order to reverse the polarization in ferroelectric, it is necessary to apply voltage between an upper electrode and the semiconductor substrate and generate an electric field in the ferroelectric. When unnecessary film is formed on an interface between a ferroelectric and a semiconductor substrate, the structure becomes a structure of a stacked capacitor. Thus, the voltage to be applied must be increased to generate a sufficient electric field in the ferroelectric.
Further, the trap level is increased due to the formation of the unnecessary film so that electrons, etc., are trapped in the interface and the film, with the result that the required property cannot be obtained.
(2) The high temperature treatment is needed.
The crystallizing temperature of oxide ferroelectric is generally high and if heating at about 600.degree. C. is not provided, for example, PZT is not crystallized. This leads to a large problem in the diffusion of Pb, etc., into the semiconductor substrate and the integrity of Si process.
(3) The crystal structure is complex and problems derived therefrom are generated.
It is necessary for oxide ferroelectric to be made a mixed crystal type having three elements or more. Since the crystal structure is complex, there arise problems such as the difficulty in film formation and working and the deterioration of crystallization due to the polarization reversal.
To solve such problems, use of fluoride (BaMgF.sub.4) as the ferroelectric layer has been proposed. In this case, the possibility of solving the above described problem (1) exists. Nevertheless, the above problems (2) and (3) cannot be solved.